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"Invisible" or "deep" Web has terabytes of information in 7500 compared to 19 terabytes of "surface" or Internet, respectively, 550 billion and 1 billion individual documents.

The data brightplanet.com - known source of information about the search for Deep Web.

Quality content "deep" Internet in 1000-2000 times higher than in the "surface" Web. First of all, it's because more than half of "deep" Web is the database and 95% of them no have mailing lists and subscriptions.